This invention relates to a multi-layer circuit board, and more particularly to a multi-layer circuit board wherein a plurality of circuit boards are superposed through a plurality of insulating layers in order and electrically connected to each other via conductive through-hole sections.
Conventionally, a multi-layer circuit board wherein a plurality of circuit patterns are laminated or superposed on each other through a plurality of insulating layers in turn is often formed with through-hole connection conductors for electrically connecting the circuit patterns to each other therethrough. For example, Japanese Patent Application Laid-Open Publication No. 194697/1990 (2-194697) discloses techniques wherein through-hole connection conductors made of a conductive material by plating each are arranged on an inner surface of each of through-holes formed so as to commonly extend through both insulating layers and circuit patterns. Other techniques for forming through-hole connection conductors are also proposed wherein a conductive paste made of resin and a conductive material such as silver particles or the like contained in the resin is applied to an inner surface of each of through-holes and then subject to calcination, resulting in a through-hole conductor being provided.
Of the conventional techniques described above, the former techniques require a step of carrying out plating of a conductive material on the inner surface of each of the through-holes. Unfortunately, the step is highly complicated and requires much time and labor. The latter techniques fail to permit an end surface of each of the circuit patterns positions inside the multi-layer circuit board to be fully contacted with the through-hole connection conductor, to thereby increase an electrical resistance between both. This would be due to the fact that the prior art causes the inner surface of the through-hole to be rendered coarse during formation of the through-hole, to thereby fail to permit silver particles contained in the conductive paste to be satisfactorily contacted with the end surface of the circuit pattern exposed to the inner surface of the through-hole. Also, the silver particles are generally formed into a scale-like shape having an average diameter as large as 10 .mu.m or more. This fails to provide a sufficient contact area between the silver particles and the end surface of the circuit pattern formed of a copper foil and positioned in the through-hole, because the circuit pattern is formed into a thickness as small as about tens of microns (.mu.m). This causes an electrical resistance between each of the circuit patterns positioned inside the multi-layer circuit board and each of the through-hole connection conductors to be as high as hundreds of ohms (.OMEGA.).